Enhanced Intersystem Crossing due to Resonant Energy Transfer to a Remote Spin.

A new mechanism for enhanced intersystem crossing in coupled three-spin systems consisting of a chromophore and an attached radical is proposed. It is shown that if the unpaired electron of the radical experiences spin-orbit coupling and different exchange interactions with the two unpaired electron spins of the chromophore, energy transfer from the chromophore to the radical can occur together with singlet-triplet intersystem crossing in the chromophore. The efficiency of this process increases dramatically when the electronic excitation of the radical is resonant with the S1-T1 energy gap of the chromophore. The types of systems in which this resonance could be achieved are discussed, and it is suggested that the mechanism could result in improved sensitization in near-IR emitting lanthanide dyes.